A process by which one or more specific reactive gas phase species (i.e., a pure gas phase species, a vapor phase species, and/or a species that is a combination of gas and vapor phases) are extracted from a gas flow (which may be a pure gas flow or an aerosol that includes suspended particles and/or droplets) by bringing the molecules of the gas phase species into contact with a reactive denuding surface through diffusion is referred to as diffusion denuding of the gas flow. Diffusion denuding can take place either passively (without moving the gas flow across the denuding surface) or actively (by moving the gas flow across the denuding surface).